Difference Between Endocrine and Exocrine Glands

Main Difference – Endocrine vs Exocrine glands

Endocrine glands and exocrine glands are two types of glands that produce and secrete chemical substances to control the functions of the body. The main difference between endocrine glands and exocrine glands is that endocrine glands are ductless glands that secrete hormones into the blood whereas exocrine glands consist of ducts and they secrete enzymes. Hormones serve as chemical messengers which regulate the physiology and behavior. Enzymes are biological catalysts, which accelerate chemical reactions. The thyroid gland, pituitary gland, and adrenal glands are the examples of endocrine glands. The gastric glands, salivary glands, and sweat glands are examples of exocrine glands. However, some glands such are liver and pancreas may serve as both endocrine and exocrine glands. These types of glands are called as mixocrine glands.

Key Areas Covered

1. What are Endocrine Glands
      – Definition, Types, Features, Functions
2. What are Exocrine Glands
      – Definition, Types, Features, Functions
3. What are the Similarities Between Endocrine and Exocrine Glands
      – Outline of Common Features
4. What is the Difference Between Endocrine and Exocrine Glands
      – Comparison of Key Differences

Key Terms: Endocrine Glands, Enzymes, Exocrine Glands, Hormones, Multicellular Exocrine Glands, Primary Endocrine Glands, Secondary Endocrine glands, Unicellular Exocrine Glands

What are Endocrine Glands

The glands that secrete hormones into the blood are known as endocrine glands. Hormones are involved in maintaining homeostasis and regulating the physiology of the body. The main physiological functions regulated by the secretions of the endocrine glands are metabolism, reproductive functioning, uterine changes, and menstrual flow. Two types of endocrine glands can be identified in the body as primary endocrine glands and secondary endocrine glands.

Figure 1: Major endocrine glands
1 – Pineal gland, 2 – Pituitary gland, 3 – Thyroid gland, 4 – Thymus, 5 – Adrenal gland, 6 – Pancreas, 7 – Ovary, 8 – Testis

Primary endocrine glands secrete hormones to regulate the secretions of the secondary endocrine glands. Pituitary gland and hypothalamus are types of primary endocrine glands which serve as the main control centers by regulating the other glands in the body. The thyroid glands, parathyroid glands, adrenal glands, and the pineal glands are the other types of primary endocrine glands. Kidneys, thymus, and gonads are secondary endocrine glands. The major endocrine glands are shown in figure 1. 

What are Exocrine Glands

Exocrine glands are a type of glands which secrete enzymes by means of a duct on the body surfaces. The most prominent exocrine glands are sweat glands, sebaceous glands, and mammary glands. Exocrine glands also store digestive enzymes in the intestine. Two types of exocrine glands can be identified based on the complexity of glands. They are unicellular exocrine glands and multicellular exocrine glands.

Unicellular Exocrine Glands

The unicellular exocrine glands comprise a specialized, single cell for storage and these cells are scattered among other non-secretory epithelial cells. The most common examples of unicellular exocrine glands are goblet cells, which are located in the epithelium of trachea and digestive tract. The goblet cells secrete mucus to the cavities.  Unicellular exocrine glands are devoid of ducts.    

Multicellular Exocrine Glands

The epithelium of the tissue invaginates to form a simple or complex tube. The tube comprises a blind end to secrete things out, and the other end blows up to form round shaped bags called alveolar or acinus. The structure of a multicellular exocrine gland with acini is shown in figure 2.

Figure 2: Multicellular exocrine gland

Three types of exocrine glands can be identified based on the mode of secretion: merocrine glands, apocrine glands, and holocrine glands. The merocrine glands secrete their own cellular products. The apocrine glands gather the cellular products on the top surface of each cell in the gland and later forms the gland’s lumen. All cells of the gland are involved in the secretion in holocrine glands.

Three types of exocrine glands can be identified based on the type of secretion. They are serous glands, mucous glands, and mixed glands. The serous glands secrete proteins, which may serve as enzymes. The mucous glands secrete mucus. The mixed glands secrete both proteins and mucus.

Similarities Between Endocrine and Exocrine Glands

• Both endocrine glands and exocrine glands produce and secrete substances to regulate the functions of the body.

Difference Between Endocrine and Exocrine Glands

Definition

Endocrine Glands: Endocrine glands are a type of glands that secrete substances (hormones) into the blood stream.

Exocrine Glands: Exocrine glands are a type of glands which release its secretion external to or at the surface of an organ with the help of a canal or duct.

Ducts

Endocrine Glands: Endocrine glands are a type of ductless glands.

Exocrine Glands: Exocrine glands may or may not have ducts.

Secretion

Endocrine Glands: Exocrine glands secrete into the blood.

Exocrine Glands: Endocrine glands pour their secretions directly at the site of action.

Type of Secretion

Endocrine Glands: Endocrine glands secrete hormones.

Exocrine Glands: Exocrine glands secrete enzymes.

Target

Endocrine Glands: The target of the endocrine glands is located away from the gland.

Exocrine Glands: The target of the exocrine glands is found very close to the gland.

Response Time

Endocrine Glands: The response of endocrine glands is delayed since the secretion should be transported through the blood to the target organ.

Exocrine Glands: Exocrine glands show a rapid response since the substances are secreted directly to the target organ.

Function

Endocrine Glands: The exocrine glands mainly control long term activities of the target organs.

Exocrine Glands: The secretions of the endocrine glands mainly control short term activities of the body.

Types

Endocrine Glands: Primary endocrine glands and the secondary endocrine glands are the two types of endocrine glands found in the body.

Exocrine Glands: Unicellular exocrine glands, multicellular exocrine glands, merocrine glands, apocrine glands, serous glands, mucous glands, and mixed glands are the types of exocrine glands found in the body.

Examples

Endocrine Glands: The thyroid gland, pituitary gland, and adrenal glands are the examples of endocrine glands.

Exocrine Glands: The gastric glands, salivary glands, and sweat glands are the examples of exocrine glands.

Conclusion

Endocrine glands and exocrine glands are the two types of glands which produce and secrete chemical substances to regulate the functions of the body. Endocrine glands mainly secrete hormones. Since endocrine glands lack ducts, the secretions of the gland are directly released to the blood. Exocrine glands secrete enzymes and mucus. Most exocrine glands are composed of ducts as well. The main difference between endocrine glands and exocrine glands is the structure and function of each gland.

Reference:

1. “Endocrine Glands.” Visible Body – Virtual Anatomy to See Inside the Human Body. N.p., n.d. Web. Available here. 24 July 2017. 
2. Darling, David. “Exocrine glands.” The Worlds of David Darling. N.p., n.d. Web. Available here. 24 July 2017. .

Image Courtesy:

1. “Illu endocrine system” Public Domain) via Commons Wikimedia
2. “Centroacinar cells” By Mikael Häggström – Basic histology – Junqueira & Carneiro, ISBN 0-07-144091-7, McGraw-Hill Education 2005 (Public Domain) via Commons Wikimedia